Reverse stressing

ABSTRACT

A METHOD AND MEANS FOR INCREASING THE FATIGUE LIFE AND LOADING CAPACITY OF A STRUCTURAL MEMBER HAVING A FIRST PORTION NORMALLY SUBJECTED TO TENSILE LOADS AND A SECOND PORTION NORMALLY SUBJECTED TO COMPRESSIVE LOADS BY EFFECTIVELY INCREASING THE LENGTH OF THE SECOND PORTION AND DECREASING THE LENGTH OF THE FIRST PORTION. IN ONE EMBODIMENT, AN OPENING IS FORMED IN THE SECOND PORTION, AND THE OPENING IS EXPANDED BY REVERSE LOADING THE MEMBER SO THAT A RETENTION MEMBER CAN BE PLACED IN THE EXPANDED OPENING. IN ANOTHER EMBODIMENT, THE SECOND PORTION IS STRUCK BY ONE OR MORE MEMBERS TO DEFORM THE SECOND PORTION THEREBY INCREASING ITS EFFECTIVE LENGTH.

' 12,1; C. J. SCHWAB REVERSE 'STRESSING :F'ilec} Feb. 75, 1968 Y 3 Sheets-Sheet PRIOR ART PRIOR ART lNl/ENTOR CHARLES .1. SCHWAB BY q, H

A T'TOR/VEYS Jan. 12, 197 c. J; SCHWAB 3,554,000

" I REVERSE STRESSING Filed Feb. 5, 1968 3 Sheets-Sheet I! Jan 12, 1971- c. J, SCHWAB 3,554,660

REVERSE STRESSING Filed Feb.- 1968 s Sheets-Sheet :5

United States Patent 3,554,000 REVERSE STRESSING Charles J. Schwab, Fox River Grove, Ill., assignor to Brook Equipment Company, a corporation of Illinois Continuation-impart of application Ser. No. 511,607, Dec. 6, 1965. This application Feb. 5, 1968, Ser. No. 703,075 Int. Cl. 1321 9/12; B23p 9/04, 13/04; B30b 1/18, I/32 US. Cl. 72453 14 Claims ABSTRACT OF THE DISCLOSURE A method and means for increasing the fatigue life and loading capacity of a structural member having a first portion normally subjected to tensile loads and a second portion normally subjected to compressive loads by effectively increasing the length of the second portion and decreasing the length of the first portion. In one embodiment, an opening is formed in the second portion, and the opening is expanded by reverse loading the member so that a retention member can be placed in the expanded opening. In another embodiment, the second portion is struck by one or more members to deform the second portion thereby increasing its eifective length.

This application is a continuation-in-part of my copending application Ser. No. 511,607, filed Dec. 6, 1965, now abandoned and assigned to the assignee of the present application.

This invention relates in general to the improvement of structural or load carrying members, and more particularly to a method and means for increasing the senvice life and loading capacity of a member which is subjected to flexural loading during normal use.

Those skilled in the art are familiar with, and well versed in the use of, punching, clamping, compressing,

and forming tools wherein such tools are provided with a frame or structural member which is subjected to the opposing forces required for the performance of the above-mentioned functions. Such devices typically employ a C-shaped frame, with the working forces being applied between the opposed ends of the frame. Oftentimes the curved portion of the C-shaped frame mentioned above is I-shaped in cross section, so that a curved I-beam is in effect defined wherein theinner or concave portion of the frame is placed 'in tension during use, and the outer or convex portion of the frame is placed in compression during use. Repetitive cycling of devices such as those described above subjects the frame portion to flexural loading, and often the frame member will fracture or fail prematurely while the remainder of the device is still in good working order. Such failure is commonly in the form of a fracture or crack that appears either at a point where machining has created a stress concentration, or in the weakest area of the frame. Heretofore, no successful means have been provided for preventing such failures, and as a result, the service life of such dew'ces was limited to 7,000 to 15,000 cycles. Since devices of the character described above often must be light in weight, so as to be portable and easy to operate, it has been impractical to materially increase the size of the frame members, and those skilled in the art have come to accept a relatively short service life as an inherent deficiency of such devices. Accordingly, the general purpose of the present invention is to provide an improved frame structure for a tool such as that described above which does not materially increase the weight of the frame, yet substantially increases the service life of the device while also increasing its loading capacity.

An object of the invention is to provide an improved frame member for a tool or the'like wherein the frame member is subjected to repeated flexural loading.

Another Object of the present invention is to produce an improved frame member as described in the preceding paragraph wherein the expected service life of the frame member is greatly increased, as from 7,000 to 15,000 cycles to in excess of 40,000 cycles such as to around 75,000 cycles, by means providing a reverse preloading.

A further object of the invention is to provide a new and improved method for increasing the service life of a structural member, such as the frame member mentioned in the preceding paragraphs.

These and other objects of the invention will hereinafter become more fully apparent from the following description taken in connection with the annexed drawing, wherein:

FIG. 1 is a side elevational view of a typical prior art device;

FIG. 2 is a central sectional view taken through the device shown in FIG. 1;

FIG. 3 is a sectional view taken as shown along line 3-3 of FIG. 1;

FIG. 4 is a fragmentary side elevational view similar to FIG. 1, and illustrating a first embodiment of the improvement of the present invention;

FIG. 4a is a fragmentary detail view illustrating an intermediate step in the method of the present invention;

FIG. 5 is a fragmentary side elevational view similar to FIG. 4, and illustrating a further embodiment of the improvement of the present invention;

FIG. 6 is a fragmentary side elevational view, similar to FIGS. 1, 4 and 5, and illustrating a still further embodiment of the invention;

FIG. 7 is a fragmentary front view, taken generally along line 77 of FIG. 6; and

FIG. 8 is a sectional view taken generally along line 8-8 of FIG. -6.

While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described in detail only specific embodiments with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the embodiments illustrated. The scope of the invention will be pointed out in the appended claims.

TYPICAL PRIOR ART DEVICE For purpose of example, the present invention has been illustrated in connection with a hydraulically actuated compression tool designated generally by reference numeral 10 in FIGS. 1 and 2, and tool 10 is conventionally utilized to crimp electrical connectors onto wires. It should be understood that the crimping tool 10 is utilized herein only as a vehicle for illustrating the principles of the present invention, and that the present invention has general application and is not limited to tools such as that shown at 10 in FIGS. 1 and 2.

The crimping tool 10 includes a frame or structural member 11 having an internally threaded bore 12 at one side thereof. As can be clearly seen in FIGS. 1 and 2, frame 11 is generally C-shaped in side elevation, and includes a'curved central portion 13, and opposed ends 14 and 15. As is best seen in FIG. 3, the central section 13 of the frame 11 is generally I-shaped in cross section, and is defined by a central web 17 and inner and outer flanges 18 and 19, respectively. It will be readily understood that the inner surface 18a of flange 18 is concave, while the outer surface 19a of flange 19 is convex.

A sleeve '20 is provided with an externally threaded portion 21 received in internally threaded bore 12, and a shoulder 212' on the sleeve 20 seats against a facing surface .23 on the frame 11. An axially extending cylindrical bore 24 is provided in sleeve 20, and an opening 25 in the outer end of the sleeve '20 communicates with the bore 24 through an inclined passage 2.6. A die carrier piston 27 is 3 mounted for movement within bore 24, and piston 27 is moved to the left as viewed in FIG. 2 when fluid under pressure is admitted from fitting 28 into bore 24 through passage 26. A piston return spring 29 bears against a spring retainer 30 to return the piston to the position of FIG. 2. The opposite end of spring 29 bears against a flange 31 on rod 32 fixed in housing member 20 as by threaded connection 33. It will be readily apparent to those skilled in the art that suitable dies, not shown, may be provided in recesses 34 in frame 11 and 35 in piston 27. Adjacent each die recess, suitable die retention means is provided as at 36 and 37. Thus, when the piston 27 moves outwardly, forces are applied to the opposite sides of the frame 11 which tend to spread ends '14 and 15 apart and place flange 18 in tension and flange 19 in compression. The tensile and compressive stresses vary from a high concentration adjacent surfaces 18a and 19a respectively to an area of zero stress within the web 17. It has been found that the induced tensile and compressive forces due to the flexural loading of the frame 11 upon repeated IMPROVEMENT OF THE PRESENT INVENTION The improvement of the present invention will be best understood from a consideration of FIGS. 4, 4a, and -8, wherein the same reference numerals utilized in FIGS. 1-3 designate corresponding elements in FIGS. 4, 4a, and 58. The improvement of the present invention will be hereafter described in terms of process or method, but it should be understood that the inventive novelty embraces both the process or method and the specific means hereafter described.

Initially, the frame 11 is examined to determine a likely area of failure, and this area can be determined by experiment, stress analysis, or by the employment of stress coating procedures which initially show areas of high stress, as is well known to those skilled in the art. A likely area of failure is shown in broken lines at 40 in FIG. 4 adjacent a machined surface 41 on the concave surface 18a of flange 18. Once a likely area of failure has been determined, a position is established in the frame portion normally under compression generally radially outwardly of the likely area of failure. The established position mentioned in the preceding sentence will hereinafter he referred to as the retention member station, and in FIG. 4 the retention member station is shown at 42.

Once the retention member station has been selected, a small hole 43 is drilled through the frame 11. In the embodiment of FIGS. 4 and 4a, the hole 43 is shown immediately radially inwardly from the flange 19 on a radial line between the likely area of failure 40 and the retention member station 42. A radial slot 44 is then cut in the frame flange 19 at the retention member station 42 through to the hole 43. The slot 44 of the embodiment disclosed in FIGS. 4 and 4a is tapered from a relatively wide portion adjacent surface 19a to a relatively narrow portion adjacent the hole 43. As will hereafter become more fully apparent, slot 44 defines holding means for reception of a retention member, while hole 43 defines a stress relief means to prevent further extension of the slot 44.

After the retention member holding means has been formed in the frame 11, the frame is placed and securely held in a suitable fixture, and the frame is reverse loaded as by the application of a force F shown by the arrow in FIG. 4. It will be appreciated that when the force F is applied, the frame ends 14 and tend to move toward one another, and the flange 18 normally loaded in tension is placed in compression, while the flange 19 normally loaded in compression is placed in tension. It will also be understood that the application of the force F slightly enlarges the slot 44. The magnitude of reverse force F is preferably approximately the same as t at encountered during normal use of the device 10, but under no circumstances should the force F exceed the yield point of the material of the frame 11. The frame 11 is maintained in the reverse loaded condition, and a retention member 45 is inserted in the retention member holding means defined by slot 44. In the embodiment of FIGS. 4 and 4a, the retention member 45 is wedge-shaped, and is forced into the enlarged tapered slot 44. Once the retention member has been inserted, the force F is removed, and the frame 11 tends to move toward its initial unloaded condition, but is prevented from moving to this position by the retention member 45. Thus, the retention member 45 defines means for precompressing the flange 18 of the frame 11, and this built-in compressive force must obviously be overcome during use of the device 10 before the frame member can be moved to its initial unstressed position and then into tension. Once the reverse loading force F is released, the retention member 45 will be tightly and securely held in place in the slot 44, and the retention member 45 serves to effectively increase the length of the flange 19, while the length of flange 18 stays substantially the same. It has been found that by using a retention member 45 in the manner described above the load carrying capacity of the frame 11 is greatly increased, and the service life is increased from approximately 7,000 to 15,000 cycles to a life in excess of 40,000 cycles.

The embodiment of FIG. 5 is substantially the same as that of FIGS. 4 and 4a, except for the use of a dilferent retention member, and retention member holding means. In the embodiment of FIG. 5, a second hole 46, slightly larger than hole 43, is formed radially outwardly of hole 43 in flange 19. A slot 47 is cut in flange 19 and extends radially inwardly from surface 19a, through hole 46 to hole 43. A cylindrical retention member 48 may be provided in hole 46 after the force F has been applied, or, if desired, hole 46 may be tapered, with a conically shaped retention member placed therein.

In an exemplary embodiment of the invention, wherein the distance between frame ends 14 and 15 is approximately one inch, the amount of deflection and return between points 14 and 15 during prior use was approximately .050 inch. When the force F is applied, and the retention member is inserted in the retention member holding means, points 14 and 15 are moved toward one another approximately .050 inch, and when the force F is released, the points 14 and 15 move away from one another approximately .020 inch leaving points 14 and 15 approximately .030 inch closer to one another. Now, during normal use, the deflection of points 14 and 15 is still approximately .050 inch but before the loading due to work is applied to the frame 11 in an amount to place flange 18 in tension and flange 19 in compression, the points 14 and 15 must first be spread the .030 inch, and thus only approximately .020 inch deflection between points 14 and 15 takes place during use in a direction placing flange 18 in tension.

Referring now to the embodiment of FIGS. 6-8, a further means is illustrated therein for increasing the effective service life of a structural member, which eliminates the need for separate insert members, such as those shown at 45 and 48 in the previous embodiments. Although the embodiment of FIGS. 6-8 will be described in terms of a crimping tool, such as that illustrated in FIG. 1, it should be appreciated that the principles of the embodiment illustrated in FIGS. 6-8, as with the earlier embodiments, apply to various different types of structural members; such as, straight or curved I-beams, a I-shaped hoist hook with or without an I-beam configuration, and other forms of structural members having a portion normally loaded in tension and a portion normally loaded in compression. Referring specifically to FIGS. 6-8, the reference characters designating the elements corresponding to elements in the earlier embodiments are primed, it being noted that the tool 10' includes a portion having an I-shaped configuration (FIG. 8) including spaced flange 18' and 19' connected by a transverse web 17'. As is best seen in FIG. 7, the structural member is prestressed by bringing opposed dies D into engagement with the opposite faces of flange 19', which is loaded in compression during normal use, to deform the material of flange '19 and form oppositely facing arcuate recesses '50. The striking of the flange 19' by the dies D, or other suitable members, displaces the material in flange 19' outwardly in the direction of the arrows in FIG. 6-, so that the length of flange 19 is effectively increased, while at the same time, the length of flange 18', which is loaded in tension during normal use, is effectively decreased.

Dies D are brought into engagement with flange 19' in an area which is generally opposite from the likely area of failure 40' in flange18. After the dies D has struck flange 19', and have moved out of engagement therewith, the material in flanges 18' and 19' has a permanent set, with the material in flange 18' being compressed so as to be prestresesd against tensile forces, and with the material in flange 19 being expanded so as to be prestressed against compressive forces. While dies D have been illusface of the flange 19' may be struck with a suitable mem her to provide a radially deformed portion in flange 19'.

In an exemplary embodiment of the invention illustrated in FIGS. 6-8, wherein the distance between the frame ends 14 and 15 is approximately one inch, the amount of deflection between points 14 and 15 is approximately .025 inch. The dies D are brought into engagement with the op posite faces of flange 19' with an appropriate force to get the deformation desired, for example, 30 tons, and recesses or dimples 50 have a depth dependent on the shape and size of the die. It has been found that crimping tools prestressed in accordance with the embodiment of FIGS. 6-8 have an effective service life of on the order of 75,000 cycles. I p

From the foregoing it will be readily apparent that the present invention provides a relatively simple, yet effective method and means for increasing the load carrying capacity and effective service life of a structural member. While the invention has been illustrated and described primarily in connection with a frame member which is a curved or C-shaped I-beam, it will be obvious to those skilled in the art that the present invention 'finds application wherever it is practical to reverse load a structural member, or to build-inf a reverse compressive force in an area normally loaded in tension.- The surprisingly improved results are accomplished without materially increasing the Weight of the structural member, and thus it will be evident that each of the objects of the invention has been fully achieved.

I claim:

1. A structural member adapted to be subjected to repeated bending loads during normal use comprising, a unitary body having a first portion subjected to a tensile force when the member is subjected to a bending load, a second portion subjected to a compressive force when the member is subjected to a bending load, said body being reversely prestressed, whereby said first portion is subjected to a compressive force and said second portion is subjected to a tensile force, means for maintaining the reverse load on said body whereby the fatigue life and loading capacity of said body is increased, said means including means defining an opening in said second portion transverse to the longitudinal axis of the tensile force, said opening being expanded a predetermined amount when a reverse load is applied, thereby to place the first portion in compression, and a retention member received in said expanded opening in compression, retaining th first portion in compression.

2. The invention set forth in claim 1 wherein said structural body is generally C-shaped and includes a curved central section and opposed ends, said repeated loads being applied between said body ends whereby the first portion of said body is at the inside of said curved section and the second portion of said body is at the outside of said curved section.

3. The invention set forth in claim 2 wherein said curved central section is generally I-shaped in cross section.

4. A structural member which is subjected to repeated loads during normal use comprising, a unitary body having a first portion subjected to a tensile force during normal use, a second portion subjected to a compressive force during normal use, said body being reverse loaded whereby said first portion is subjected to a compressive force and said second portion is subjected to a tensile force, means for maintaining at least a portion of the reverse load on said body whereby the fatigue life and loading capacity of said body is increased, said means including means forming a tapered opening in said second portion, said opening expanding a predetermined amount when said reverse load is applied, and a retention member wedged in said expanded opening.

5. The method of increasing the service life and loading capacity of a unitary structural member which is subjected to flexural loading during normal use comprising the steps of providing an opening in a portion of the member that is placed in compression during normal use; subjecting the member to reverse loading whereby portions normally placed in tension are placed in compression, and portions normally placed in compression are placed in tension so that said opening becomes enlarged; and retaining the member in at least a partially reverse loaded condition by placing a retention member in said enlarged opening and releasing the reverse loading on said member to thereby induce stresses in said member which must first be overcome during use of the member before stresses resulting from normal loading are applied to the member.

6. The method of claim 5 including the initial step of determining a likely area of failure, and the further step of selecting a location for said opening so as to minimize the possibility of failure at said likely area.

7. The method of increasing the service life and loading capacity of a unitary structural member which is subjected to flexural loading during normal use so as to place a portion of the member in compression and a portion of the member in tension, said method comprising the steps of: determining a likely area of failure; selecting a location for reverse loading the member so as to minimize the possibility of failure at said likely area; and prestressing said member by effectively increasing the length of the portion normally placed in compression to effectively decrease the length of the portion normally placed in tension to thereby induce stresses in said member which must first be overcome during use of the member before stresses resulting from normal loading are applied to the member, said prestressing step being performed by providing an opening in the portion of the member that is placed in compression during normal use, subjecting the member to reverse loading to expand the opening, inserting in said expanded opening a retention member having a dimension greater than that of the unexpanded opening, and releasing the reverse loading on said member.

8. The method of increasing the service life and loading capacity of a unitary structural member which is subjected to flexural loading during normal use so as to place a portion of the member in compression and a portion of the member in tension, said method comprising the steps of: determining a likely area of failure; selecting a location forreverse loading the member so as to minimize the possibility of failure at said likely area; and prestressing said member by effectively increasing the length of the portion normally placed in compression to effectively decrease the length of the portion normally placed in tension to thereby induce stresses in said member which must first be overcome during use of the member before stresses resulting from normal loading are applied to the member said prestressing step being performed without adding material by deforming the portion of the member that is normally placed in compression by application of force of a compressive tool defining a permanent depression in the material.

9. A unitary structural member comprising: a first portion subjected to tensile force during use; a second portion subjected to compressive force during use; and means for maintaining a reverse load on both portions of said member to increase the service life and loading capacity of the member Without adding material, said means including permanently displaced material in said second portion for effectively placing said portion normally in tension and effectively placing said first portion in compression, the material in the second portion being displaced as a result of permanent compressive deformation of the second portion by application of force of a compressive tool defining a permanent dimple in the material.

10. The invention set forth in claim 9 wherein the structural member has a cross section of generally I-shane, including a relatively thin midportion, a relatively wide edge portion comprising said first portion, and a relatively wide edge portion comprising said second portion, and the material in the second portion is displaced by permanent deformation of the second portion by application of force of a compressive tool defining a permanent dimple in the material.

11. The invention set forth in claim 9 wherein said structural member is generally C-shaped and includes a curved central section and opposed ends, said loads being applied between said member ends whereby the first portion of said member is at the inside of said curved section and the second portion of said member is at the outside of said curved section.

12. The invention set forth in claim 11 wherein the structural member has a cross section of generally I shape, including a relatively thin midportion, a relatively Wide edge portion comprising said first ortion, and a relatively wide edge portion comprising said second portion, and the material in the second portion is displaced by permanent compressive deformation of the second portion by application of force of a compressive tool defining a permanent depression in the material.

.13. A compression tool for applying repeated loads during normal use comprising, a unitary body having a first portion subjected to a tensile force during normal use, a second portion subjected to a compressive force during normal use, said unitary body being generally C-shaped and including a curved central section and opposed ends, one of said body ends having a recess for a fixed compressive die, said other body end comprising a housing having a cylinder, a piston movable in the cylinder including a recess for a compression die movable toward and away from the fixed die, and a fitting for admitting motive fluid to the cylinder, said repeated loads being applied between said body ends whereby the first portion of said body is at the inside of said curved section and the second portion of said body is at the outside of said curved section, said body being reverse loaded whereby said first portion is subjected to a compressive force and said second portion is subjected to a tensile force, means for maintaining at least a portion of the reverse load on said body whereby the fatigue life and loading capacity of said body is increased, said means including means defining an opening in said second portion, said opening expanding a predetermined amount when said reverse load is applied, and a retention member received in said expanded opening.

14. A compression tool for applying repeated loads during use, comprising; a unitary structural member having a first portion subjected to tensile force during use; a second portion subjected to compressive force during use; said structural member being generally C-shaped and including a curved central section and opposed ends, one of said member ends having a recess for a fixed compressive die, said other member end comprising a housing having a cylinder, a piston movable in the cylinder including a recess for a compression die movable toward and away from the fixed die, and a fitting for admitting motive fluid to the cylinder, said loads being applied between said member ends whereby the first portion of said member is at the inside of said curved section and the second portion of said member is at the outside of said curved section; and means for maintaining a reverse load on both portions of said member to increase the service life and loading capacity of the member, said means including permanently displaced material in said second portion for effectively placing said second portion normally in tension and effectively placing said first portion in compression.

References Cited UNITED STATES PATENTS 1,987,957 1/1935 Kerns et al 29-257 2,039,398 5/1936 Dye 29-446 2,696,729 12/ 1954 Vander Heyden 52-229 3,101,272 8/ 1963 Setzer 29-446 3,166,830 1/1965 Greulich 29-446 3,210,837 10/1965 Hassellof 29-446 3,294,608 12/1966 Peterson 29-446 1,580,894 4/1926 Hummel 100-214 2,887,762 5/1959 Dobell 29-155 FOREIGN PATENTS 517,995 3/1953 Belgium 52-227 993,885 11/1951 France 52-229 CHARLES W. LANHAM, Primary Examiner E. M. COMBS, Assistant Examiner US. Cl. X.R.

53 55 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Dated January 12, 1971 Patent No. 315541000 Inventor(s) Charles J Schwab It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Correct the Assignee in the patent heading to read:

-Brock Equipment Compan Signed and sealed this 1st day of June 1971.

(SEAL) Attest:

EDJARD M.FLETCHER,JR. WILLIAM E. SGHUYLER, JR. Attesting Dfficer Commissiorer of Patents 

